Miniature breadboard voltmeter

Dec 13

In this article, I show you a cool little tool that can save you from headaches. The project is about a miniature, 2-channel voltmeter that you can directly plug in into a breadboard. It intends to decrease the clutter when you develop a circuit on a breadboard and you need to measure the voltage at different points of the circuit. I personally found it very useful to measure the output voltage of potentiometers.

Introduction

The project originally stems from my CH32 tutorial series. I started working with this chip not so long ago and started to create projects and tutorials so that more people can get access to well-explained resources. During the tutorial series, I discovered the ADC of the CH32V003F4P6 chip and I also wrote a custom library to drive different OLED displays. When I was working on the ADC-related tutorial, I generated the voltage to be measured by the ADC of the microcontroller via a potentiometer. To confirm the measured voltage by the ADC, I used one of my multimeters. It was a bit cumbersome because of the wires and the lack of space on my computer desk. Then it occurred to me that it would be nice to have a tiny voltmeter that I can have directly in the breadboard.

This shifted my attention to the CH32V003J4M6 microcontroller which is an 8-pin microcontroller. It can be considered as the “direct competitor” of the ATtiny85 microcontroller. However, the CH32 is somewhat more powerful, so maybe we cannot really talk about direct competition. :) Also, to go down in size as much as possible, I selected a 0.42”, 72x40 pixel OLED display for this project. The PCB for the circuit is only 12 mm wide and 24 mm long, so it is tiny. So the whole circuit only takes up 4 rows of space from a breadboard. Its minimal occupancy becomes even more apparent when the board is used in “standing mode”.

Features

So, as I said in the introduction, the board is driven by the CH32V003J4M6 microcontroller. There’s a minimal amount of components involved: the microcontroller, a 5 V → 3.3 V LDO (MCP1700) and its two capacitors, four resistors (2 for each ADC channel as voltage dividers), and the OLED display.

The board operates at 5 V, but the chip and the OLED are run at 3.3 V. This is a conscious choice because I thought it makes more sense to default to 5 V because many breadboard projects still use Arduino or similar microcontrollers where the default supply voltage is 5 V. In fact, I could have used 5 V for the microcontroller and the OLED display, but I felt safer to use it at 3.3 V. Regarding “safety”, I added a voltage divider to each input of the ADC channels. I picked R1 and R2 so the maximum voltage can be 12 V. This should be more than enough for breadboard projects.

The programming of the microcontroller can be done by the SWIO pin of the microcontroller. I placed 3 tabs on the backside of the PCB which are 2.54 mm apart. So basically three regular jumper pins can be used as “pogo pins” for programming. I wanted to keep it simple. Actually, when I programmed the circuit, I put it in a breadboard, drove it with 5 V and I just simply touched the wire coming from the programmer to the SWIO tab. It worked without any issues.